Function yield_basic_work

tests/system_test/yield_test.cpp:22–26

std::atomic<int> g_yield_ret{-1};

void yield_basic_work(void* /*arg*/) {
  int ret = sys_yield();
  g_yield_ret.store(ret);
  sys_exit(0);
}

/// Test 2: yield preserves execution order fairness
std::atomic<int> g_a_count{0};